1. Field of the Invention
The present invention relates to the transformation of the Guayule plant, Parthenium argentatum. 
2. Description of the Art
Natural rubber is a strategic raw material. Approximately 40,000 different products, to include at least 400 different medical devices, require natural rubber. Modern aircraft tires, for example, are not manufactured without natural rubber. Synthetic rubber can be made, but it is an imperfect substitute. The difference is apparently due to natural proteins and certain biological catalysts that occur in natural rubber, but which have not yet been reproduced in a synthetic version. Natural rubber, therefore, is of significant economic importance and having an uninterrupted supply of rubber is critical to the nation's economy. Moreover, as the industrial capacity of developing countries continues to build, the demand for natural rubber builds with it.
Natural rubber is a hydrocarbon polymer of cis-1,4-polyisoprene with 400-50,000 isoprene monomeric units enzymatically linked in a head-to-tail configuration. This head-to-tail configuration is apparently critical to the properties of natural rubber, and something that is not reliably reproduced in the synthetics.
Traditionally, virtually all natural rubber used to manufacture products has come from the Hevea brasiliensis plant which is native to tropical areas outside the continental United States. At the present time, most Hevea production is occurring in parts of the world whose political or economic stability is in question. This has implications for the rubber industry—a steady and predictable supply of rubber cannot be guaranteed from foreign sources. Moreover, because rubber production is currently overseas, shipping costs are a significant factor for users of rubber in the United States.
Many plant species other than Hevea produce rubber, although not in equal quality or in a form that is easily harvested. These plants include, but are not limited to, Euphorbia tirucalli, Taraxacum Kok-saghyz (Russian dandelion), Cryosothamnus nauseosus (rabbit brush), Asclepias incarnate, Soldiago altissima, Cryptostegia grandiflora (rubbervine), Sonchus arvensis, Cacalia atriplicifolia, Ficus elastica (Indian rubber tree), and Parthenium argentatum (Guayule).
Guayule is a particularly promising candidate for the rubber industry in North America because it is native to the Chihuahuan desert of Texas and Mexico, and because the rubber harvested from it shows unusual hypoallergenic properties. Having a local supply of rubber for manufacturers and consumers in North America, therefore, is a quite positive economic development. No longer will the domestic rubber industry be dependent on foreign sources of rubber, often from countries that may not always be dependable in terms of providing a steady source of rubber to manufacturers in the United States.
Moreover, the fact that rubber from Guayule can be used to produce hypoallergenic latex is especially significant. Rubber or latex allergy is a significant problem that affects individuals in many different groups such as health care workers, those who work in laboratories, medical and dental patients, food-service workers, and anyone else who desires the safety and convenience of rubber gloves or appliances. Latex itself is a suspension of rubber particles in aqueous media, and considerable efforts have been expended to isolate and extract latex from various sources.
Extraction of latex rubber from Guayule has previously been described by Cornish in U.S. Pat. No. 5,590,942 (1996) and U.S. Pat. No. 5,717,050 (1998), both entitled Hypoallergenic Natural Rubber Products from Parthenium argentatum (Gray) and other non-Hevea brasiliensis Species. These patents describe isolation and purification procedures, and also provide considerable background information regarding the need for hypoallergenic latex.
Producing hypoallergenic latex from plants that can be grown within the climate zones of the continental United States is highly desirable and economically quite important. Perfecting the techniques for growing Guayule and increasing the latex-yield per plant is critical to industrial-level production. Central to this process is the transformation of Guayule lines, or the introduction of various genes into the Guayule plant that will operate to boost latex yield, increase its hardiness, or expand its range.
Transformation of Guayule is dependent on a reliable regeneration system. The most common regeneration system that has been used thus far involves shoot node inoculation by Agrobacterium tumefaciens which has a low efficiency rate, is enormously time consuming, labor intensive, tedious, and therefore not very practical.
The most highly-refined technique using the shoot node culture system is apparently that described in Pan et al. (1996) which uses a Agrobacterium tumefaciens mediated-transformation system based on shoot nodes as explants. This method, using the tip of a syringe needle, involves gently wounding each node at the axillary position, and introducing Agrobacterium, which harbors the vector containing the Agronomically-important genes. The method requires each and every node to be wounded by hand, one at a time. Furthermore, the transformation efficiency rate for this technique is extremely low, 0.71% or less. In other words, the existing method for transformation of Guayule is time consuming, tedious, and inefficient.
Other transformation methods have been attempted with varying degrees of success—transformation is highly species-specific and genotype-dependent. Stated differently, what works well in one species often does not work well, or at all, in another species. Moreover, the species that are amenable to existing transformation techniques are not always those with high commercial value such as latex-producing Guayule.
What is needed, then, is a method of transformation for Guayule that is efficient and economical.